The present invention relates to centrifuges capable of separating solid and liquid components of a slurry, and more particularly, to centrifuges of the type that include a perforated drum or basket with a filter cloth which is insertable therein and from which the drum may be pulled away to invert the filter cloth to effectuate a clean discharge of the filter cake into a receiving hopper.
In the production of numerous chemical products such as fertilizers and pharmaceuticals, solid particles are suspended in a liquid vehicle to form a slurry. It is necessary to remove the liquid vehicle in the most efficient manner. Vacuum techniques for drawing away the liquid are inherently inefficient. The use of heat to vaporize the liquid consumes too much energy, and in many cases, will adversely effect the chemical compounds present in the suspended particles. Accordingly, centrifuges are widely used to spin the liquid out of slurries in many chemical and pharmaceutical processing operations.
U.S. Pat. No. 3,623,613 of Quetsch granted Nov. 30, 1971 discloses a centrifuge for separating liquid and solid components from a suspension, wherein the centrifuge is provided with a centrifugal drum and a cloth filter insertable therein. A boundary edge of the filter is attached to the discharge end of the centrifugal drum and the other boundary edge is attached to a mushroom shaped bottom piece or disc. The centrifugal drum and bottom piece are arranged co-axially with respect to each other and are rotatable together. In addition, the drum and bottom piece are relatively axially moveable with respect to each other during rotation for inverting the filter. A lid is mounted to the bottom piece and contacts the discharge end of the drum when the bottom piece is located at the bottom of the centrifugal drum during the centrifuging process. The lateral walls of the centrifugal drum are perforated to enable the passage of liquid. Slurry is gradually introduced into the drum and the centrifugal force causes the liquid therein to pass through the filter cloth and through the perforated drum. However, suspended particles in the slurry soon occupy the interstices between the fibers of the filter cloth. Accordingly, as additional slurry is introduced the built up solids on the filter cloth themselves facilitate the separation of the liquids and solids in the slurry by a process known as "cake filtration." At the appropriate time, the drum 16 and bottom piece 23 are axially separated by axial movement of shaft 12 by piston 34 while they are still rotating together. This action serves to invert the filter cloth which is connected between the drum and bottom piece for discharge of the filter cake therein. The result is a clean discharge of the filter cake into a receiving hopper for transport to further processing. The drum and bottom piece are then moved axially together to reinsert the filter cloth against the interior walls of the drum so that another filtration cycle can begin.
The foregoing type of inverting filter cloth centrifuge typically operates at high RPM, such as 2000 RPM, in order to maximize the efficiency of the separation process. The interior diameter of the drum may measure, for example two feet across. Accordingly, the centrifugal forces exerted on and its supporting structures and rotating shaft are substantial. Therefore, reliable and precise balance and support of the rotating structures are critical to an effective and economical machine.
The liquids separated from the slurries are often acids or other corrosive compounds that can attack the bearings and their protective seals. It is critical that the shaft support bearings be well lubricated and well protected from chemical attack. The seals that protect the bearings must also insure that lubricant does not leak into the product being processed.
The filter cloth is inverted for discharge or recovery of the filter cake or solids while it is rotating at high RPM. This is accomplished by the axial shifting of rotating shafts and certain supporting structures. Therefore the axially moveable and rotating shafts must be well balanced and sealed at all positions in their axial movement.
The prior art structures disclosed in the above and in the following patents incorporate the shifting cylinders of the shifting mechanism directly in the axially shifting shafts. While this has some advantages as to compactness, it is expensive and makes bearing support and sealing more difficult.
U.S. Pat. No. 4,193,874 of Gertels granted Mar. 18, 1980 is directed to and relates to the use of a support screen 41 (see FIG. 2) disposed between the filter cloth 22 and the side wall of the drum in a centrifuge of the foregoing type. The base plate is shifted outward by axial shifting of shaft 12 and its extension 33 by piston 34 as in the prior patent.
U.S. Pat. No. 3,623,613 of Quetsch, granted Nov. 30, 1971, discloses an inverting filter centrifugal separator having a hollow shaft 4 and supporting sleeve 2 which shift axially to pull the drum away from the base plate or disc for inverting the filter. The hollow shaft and sleeve serve as hydraulic pistons for the axial shifting. Thus high pressure hydraulic fluid is applied directly to the end of the shaft for shifting it. This results in the aforementioned difficulty of effectively sealing the bearings and other critical components against liquids in the system.
Accordingly it desirable that effective and reliable bearing support and sealing be provided in an inverting filter centrifugal separator.